If I select DirectX as the rendering API and make sure that the game is in full screen, I get a black screen.
Mouse movement is broken on XWayland. PROTON_ENABLE_WAYLAND=1 fixes it. But it is available on either GE or CachyOS Proton only...
On non-fullscreen mode using XWayland it also works normally...
The details is in this post on Reddit:
https://www.reddit.com/r/linux_gaming/comments/1ugdlo9/xwayland_always_breaks_something_proton_enable/
PROTON_ENABLE_WAYLAND=1x1 2026-06
Compatibility Report
System Information
Symptoms
Using OpenGL, the game runs, but high resolutions (2048x1536) will cause my mouse movements to spin rapidly by itself. It seems the game does not go fullscreen, as it has a border. High resolutions also crop the mouse cursor to a smaller dimension, making it impossible to click any buttons on the outer edges of the screen. This happens with both DXVK and WineD3D.
OGL LOG:
steam-41060ogl.log
D3D LOG:
steam-41060d3d.log
Using Direct3D causes a black screen, whether or not DXVK or WineD3D is used.
WINED3D OGL LOG:
steam-41060wineogl.log
WINED3D D3D LOG:
steam-41060wined3d.log
Same behavior as Serious Sam Classic: The First Encounter.
https://github.com/ValveSoftware/Proton/issues/3670